Image-forming apparatus, such as copiers and printers, are known for forming multiple toner images in register with each other on a substrate in sheet form. Each such toner image is referred to herein as a "color plane". Such an apparatus may include an endless belt and a number of processing stations arranged along its path. These stations may include an imaging station at which a plurality of electrostatic images are sequentially formed on the belt. At a number of developing stations, the electrostatic images are developed into toner images on the belt. The toner images are then transferred to a substrate at a number of transfer stations. The endless belt is driven along a belt path through the imaging station, the developing stations and the transfer stations by a belt drive device. By use of a substrate drive device, the substrate is driven along a substrate path into contact with the endless belt at the transfer stations.
For example, U.S. Pat. No. 4,751.549 (Koizumi/Ricoh Company Ltd) describes a color copying machine for forming multiple toner images in register with each other on a substrate in sheet form. The apparatus comprises an endless belt, an imaging station at which a plurality of electrostatic images are sequentially formed on the belt, a plurality of developing stations at which the electrostatic images are developed into toner images on the belt, a plurality of transfer stations at which the toner images are transferred to a substrate, and a substrate drive device for driving the substrate along a substrate path into contact with the endless belt at the transfer stations.
In such an apparatus, it is important to the quality of the printed image, to maintain the photoconductive belt at a constant speed. The speed of the belt is a very important parameter in controlling the length of the distinct color planes written on the belt and transferred to the substrate as well as in the registration accuracy of the transfer of succeeding color planes to the substrate in different transfer stations. While a drive motor provided to drive the belt drive roller may be controlled in a known manner to run at a constant speed, any variation in the path followed by the belt will result in variations in the belt speed at the image exposure station and at the image transfer stations. Such variations in belt path can be caused by non-perfectly circular guide rollers.
Image forming apparatus which are designed to make two images in one pass on each side of the receiving substrate (so-called simultaneous duplex engines), may consist of two imaging systems as described above with two endless belts. In such apparatus the image receiving substrate is transferred from the first imaging system to the second imaging system and consequently it is very important that both image forming members in the succeeding transfer stations are running at the same speed.
For example, International patent specifications Wo 98/07073 and WO 98/07072 (Agfa-Gevaert NV) describe a duplex image-forming apparatus for forming multiple toner images in register with each other on a substrate in sheet form. The apparatus comprises a first endless belt, a first imaging station at which a plurality of first electrostatic images are sequentially formed on the first belt, a plurality of first developing stations at which the first electrostatic images are developed into first toner images on the first belt, a plurality of first transfer stations at which the first toner images are transferred to a substrate in sheet form, and a first belt drive device for driving the first endless belt along a first belt path through the first imaging station, the first developing stations and the first transfer stations. The apparatus also comprises a second endless belt, a second imaging station at which a plurality of second electrostatic images are sequentially formed on the second belt, a plurality of second developing stations at which the second electrostatic images are developed into second toner images on the second belt, a plurality of transfer stations at which the toner images are transferred to the substrate, a second belt drive device for driving the second endless belt along a second belt path through the second imaging station, the second developing stations and the second transfer stations; and a substrate drive device for driving the substrate along a substrate path into contact with the first endless belt at the initial one of the first transfer stations and the second transfer stations.
It is an object of the present invention to provide an image forming apparatus of the type described above in which the maintenance of a constant belt speed can be more reliably assured.